Losing a single species from an ecosystem may have large effects on community and ecosystem properties, but this may depend on characteristics of the species and the ecosystem. We examined the effect of losing a single species on productivity and nitrogen retention in experimental grassland communities, concentrating on how these effects varied with the functional identity of the species lost and the diversity and composition of the community from which it was lost. In one experiment, we constructed random plant assemblages that varied in species richness to measure the effect of diversity alone on productivity and nitrogen retention. In another experiment, we constructed plant assemblages to assess the effects of deleting an individual plant species from assemblages differing in their functional and species richness and composition. On average, as species richness declined, productivity decreased but nitrogen retention was unaffected. However, the magnitude and direction of change in ecosystem functioning with declining diversity depended on the identity of the species deleted and the composition of the community from which it was deleted. The functional identity of a species predicted the type of impact its loss had on productivity, but not on nitrogen retention.